There's no bonafide way to do this. There is, however, a simple javascript "disable back-button" script available, probably on javascripts.com. But since people can turn off javascript, it's not a viable solution.

Unless you checked for javascript before the script began to process, and sent them to an error page telling them that javascript had to be enabled before continuing. Those things annoy the hell outta me, but I'm seeing more and more of them.

If the script has finished processing, and they're at a "success" page, you could make your success page a redirect to a 2nd success page. Then if they try to click back, they go back to the redirect page and get redirected all over again -- they just loop over and over.